Recently, in the field of automobiles, lightweight and high performance were demanded simultaneously. Especially, the engine components are required to be light in weight.
For example, steel or nickel alloys are used as engine valves. However, these materials are so dense that the inertial mass of the engine valves is large, which hinders an engine from rotating at high speed. By making the engine valves lightweight, the inertial mass of the valves can be reduced. Thus, the engine can be rotated at high speed and an automobile with high performance can be provided. To meet such needs, a Ti--Al system intermetallic compound is highly demanded as a light-weight, heat-resistant material, because this compound has low density and superior strength at high temperature. For such reasons, the compound has been widely researched and developed.
However, the practical use of the Ti--Al intermetallic compound is hindered for the following three reasons:
(1) the compound has an insufficient ductility at room temperature; PA1 (2) the compound is hard to work, thereby difficult to shape into components; and PA1 (3) the component has an insufficient oxidation resistance. PA1 (1) Research Report of the 123rd Committee on Heat-resistance Material, Japan Society for the Promotion of Science Vol.29 No.1 (1988), P77-87, in which Tsurumi et al. proves the improvement by the addition of niobium; PA1 (2) Minute of Autumn Conference of the Japan Institute of Metals; Journal Vol.29 (1990), P274, in which Anada et al. proves the improvement by the addition of molybdenum; PA1 (3) Minute of Autumn Conference of the Japan Institute of Metals; Journal Vol.30 (1991), P561, in which Anada et al. proves the improvement by the addition of tungsten; and PA1 (4) Minute of Autumn Conference of the Japan Institute of Metals; Journal Vol.54 No.8 (1990), P948-P954, in which Kasahara et al. proves the improvement by the addition of silicon. PA1 (1) The amount of the halogen elements is limited as aforementioned, because if the amount is less than the lower limit, an ununiformly thick aluminum oxide film is formed, and if the amount exceeds the upper limit, an imperfect aluminum oxide film is formed. In both cases the oxidation resistance is insufficient. PA1 (2) The amount of oxygen is limited as aforementioned, because if the amount is less than the lower limit, only a thin aluminum oxide film is formed and the oxidation resistance is insufficient. PA1 (3) The treatment temperature is limited as aforementioned, because if the temperature is lower than the lower limit, only a thin aluminum oxide film is formed. If the temperature exceeds the upper limit, the generation of the alpha phase, i.e., alpha solid solution of titanium alloy containing small amounts of aluminum, which tends to be easily oxidized, results in an ununiformly thick aluminum oxide film. In both cases the oxidation resistance is insufficient.
To solve these problems, the Ti--Al system intermetallic compound has been widely and variously researched and developed.
It is now clarified that the addition of manganese, chromium, vanadium or the like can improve the ductility at room temperature of the intermetallic compound.
Japan Laid-open Patent Application No. 1-30898 proposes an improvement in the difficulty in working the Ti--Al system intermetallic compound. The proposed Ti--Al system intermetallic compound is prepared by a reaction sintering process. Specifically, titanium or titanium alloy powder is first mixed with aluminum or aluminum alloy powder, and is then degassed and charged through a vacuum. Subsequently, the mixed powder is plastically deformed at the reacting synthesis temperature or lower temperature. Thus obtained mixed body is heated to the reacting synthesis temperature or a higher temperature.
The improvement in the oxidation resistance of the Ti--Al system intermetallic compound is researched and developed, as seen in the following documents:
However, such improvements in oxidation resistance are insufficient for the practical use of the Ti--Al system intermetallic compound.
The components disposed around an engine are mostly sliding members. If such components are made of titanium aluminide, they are easily worn, which is a problem in practical use. Lightweight and at the same time wear resistant sliding members are expected.